Rotor adapters are used to connect the downhole end of a mud motor to the drive shaft which turns the drill bit. The downhole end of the mud motor rotates eccentrically. That is, the end of the mud motor is displaced from a central axis and rotates around it. The rotor adapter is used to convert the eccentric motion to circular motion in which the axis of the driven shaft remains in one place.
As particularly used in downhole drilling, the rotor adapter must sustain high thrust forces. In addition, because backing out of the hole is expensive, it is desirable that the rotor adapter be able to function under high stress for as long as possible.
In addition, the rotor adapter must be able to sustain the high torque required for drilling, preferably including the extreme torque experienced when the drill bit becomes stuck.
Previously, these requirements were met by a rotor adapter having a rotor adapter shaft with ball and socket joints on either end for connecting to the drive shaft and driven shaft. However, the ball and socket joints were torch cut from a single bar of metal and required considerable thickness to have the necessary strength for drilling.
Recent developments in downhole drilling have included the adoption of an adjustable bent housing which permits adjustment of the angle of drilling at the rig site without removing the bent housing and replacing it with another. However, the adoption of the adjustable bent housing has required that the rotor adapter shaft be thinner than that permitted by the strength limitations of the ball and socket rotor adapter.
Since previous rotor adapters, particularly the ball and socket rotor adapter, were not capable of being used with the adjustable bent housing, there has been a need to develop a new rotor adapter with universal joints which is strong, long wearing and relatively thin. Previous attempts to provide such a rotor adapter have included a device which rotated around two perpendicular axes on a central block. This device was unsatisfactory due to its poor strength. The present invention, by contrast, provides a strong and long wearing rotor adapter suitable for use with the adjustable bent housing.